The present invention relates to a method of manufacturing a lead frame in which an inner lead is connected to an end of an outer lead through an etching stopper portion made of a metal material different from that of the outer lead.
FIG. 1 of the accompanying drawings shows a lead frame which comprises an outer lead 41 made of copper, an etching stopper portion 42 made of aluminum formed on the surface of the base portion, a thin inner lead 43 made of copper connected to the etching stopper portion 42 and a bump 44 made of aluminum or the like connected to the tip end of the inner lead 43. When in use, this lead frame is generally bonded to an electrode pad 46 of a semiconductor chip 45 through the bump 44 and the semiconductor chip 45 is sealed by a sealing resin 47.
When such lead frame is manufactured, as shown in FIG. 2, there is used a cladding member 511 of trilayer structure in which a thick outer lead copper layer 58, an etching aluminum layer 59 and a thin inner lead copper layer 510 are laminated. Specifically, in the cladding member 511, the outer lead copper layer 58 is selectively etched and the inner lead copper layer 510 is selectively etched, thereby resulting in a lead frame being manufactured. In this manufacturing process, since the etching stopper aluminum layer 59 is provided, the outer lead can be prevented from being corroded by the inner lead etching and the inner lead can be prevented from being corroded by the outer lead etching.
However, when such lead frame is manufactured, the patterning of the outer lead 41 should be inevitably formed by one-side etching so that a side etching amount is inevitably increased. As a consequence, each lead cannot be fine pitched and a demand for ultra-high integration cannot be satisfied sufficiently. That is, while the width of the inner lead and the spacing thereof are about 75 .mu.m, the width and spacing of the inner lead should be about 35 .mu.m in accordance with the demand of ultra-high integration. A method in which the inner lead is formed by etching cannot cope with the above demand, and it is very difficult to manufacture such lead frame from a manufacturing process standpoint.
Recently, in order to solve the aforesaid problem, there is provided a method in which the inner lead 43 is not directly formed by etching the inner lead copper layer 510 of the cladding member 511 but the inner lead is formed by plating copper on the inner lead copper layer 510 by using a resist film as a mask.
However, in the method of making an inner lead by plating copper on the inner lead copper layer 510, the above copper layer 510 is removed by etching after the inner lead was formed by plating. In that case, the inner lead is similarly made of copper so that the inner lead should be left and only the copper layer 510 should be etched. To carry out such fine etching, an exclusively-used expensive soft etchant (e.g., mixed liquid of hydrogen peroxide, sulfuric acid and fluorine acid) becomes necessary, which makes the resultant lead frame expensive. In addition, even when such soft etchant is used, the condition of such etching process, e.g., etching rate is difficult to be managed. If such management of etching rate is insufficient, there is then the risk that the inner lead also is etched. Therefore, this method is not sufficient from a standpoint of quality stability.
Further, since such soft etching process is difficult to be managed, the thickness of the copper layer 510 which exerts a large influence on this process is required to have a sufficiently high accuracy. As a result, the method of manufacturing lead frame becomes difficult.
Furthermore, in the above-mentioned method, when the inner lead is processed by electrolytic plating, an abnormal precipitated portion tends to be formed on the inner lead precipitated by the electrolytic plating. Moreover, since there is provided an etching stopper aluminum layer 59, most of the aluminum layer 59 is removed by etching, and hence the thickness of the aluminum layer 59 cannot be increased more than is necessary from a manufacturing cost standpoint and an etching process standpoint. Therefore, when the outer lead copper layer 58 is selectively etched, the aluminum layer 59 cannot function sufficiently as a barrier (etching stopper layer). As a consequence, a failure in manufacturing such as disconnection or broken-off tends to occur in the inner lead provided on the side opposite to the outer lead copper layer 58.
Since the failure in manufacturing tends to occur, yield is very low, which causes a manufacturing cost to be increased. Therefore, it is desired to prevent the failure in manufacturing.